1. Field of the Invention
The present invention relates to a cathode-ray tube, and more particularly to an electron gun for use in a cathode-ray tube.
2. Description of the Prior Art
Recently available color picture tubes employ deflection electrodes of the self-convergence type.
As shown in FIG. 1 of the accompanying drawings, such a self-convergence deflection electrode produces a horizontal deflecting magnetic field with pin-cushion distortion and a vertical deflecting magnetic field with barrel distortion for deflecting and automatically converging three R, G, B electron beams on a phosphor screen.
Since, however, the horizontal and vertical deflecting magnetic fields are distorted in the shapes of a pin-cushion and a barrel, respectively, the spot which is produced by the electron beams on the phosphor screen tends to be defocused or distorted at outer edges of the screen, as shown in FIG. 2 of the accompanying drawings. The electron beam spot is distorted because each of the electron beams which has a certain finite spatial extent is subjected to different forces in different locations on the phosphor screen.
The electron beam spot distortion, at an X-axis end of the phosphor screen, in the horizontal deflecting magnetic field which is distorted in a pin-cushion pattern will be described in greater detail with reference to FIG. 3 of the accompanying drawings. In FIG. 3, an electron beam e passes through the sheet of FIG. 3 in a direction away from the viewer, and four 90.degree.-spaced points A, B, C, D are assumed to be on the peripheral edge of a cross-sectional plane across the electron beam e. Since the magnetic field is stronger in the point B than in the point A, the electron beam e undergoes lateral pull on its opposite sides. At the same time, forces directed toward the center of the electron beam e are applied to the points C, D.
Therefore, the electron beam spot on the phosphor screen is slightly underfocused, i.e., would come to a focus beyond the phosphor screen, in the horizontal direction, and is strongly overfocused, i.e., comes to a focus short of the phosphor screen, and hence diverges to produce a halo, in the vertical direction. FIGS. 4A and 4B of the accompanying drawings schematically show, using an optical lens system simulating the electron gun, how the electron beam is focused at the center and the X-axis end, respectively, of the phosphor screen, the optical lens system including a main lens 31 and a deflection electrode 32. In FIGS. 4A and 4B, the electron beam is emitted from an object point a on a cathode, and is focused at a focus point f. The vertical lens effect of the optical lens system is shown on the upper side of a Z-axis, and the horizontal lens effect of the optical lens system is shown on the lower side of the Z-axis. The above horizontally underfocused and vertically overfocused condition of the electron beam spot is illustrated in FIG. 4B.
The relationship between the size of the electron beam spot and the focusing voltage applied to the deflection yoke is shown in FIGS. 5A and 5B of the accompanying drawings.
At the center of the phosphor screen, as shown in FIG. 5A, focusing voltages Vfv, Vfh applied to bring the electron beam spot into focus vertically and horizontally are equal to each other. The minimum sizes of the electron beam spot in the vertical and horizontal directions are the same. Therefore, the electron beam spot is substantially circular in shape at the center of the phosphor screen.
At the X-axis end, however, the focusing voltage Vfv applied to focus the electron beam spot vertically is higher than the focusing voltage Vfh applied to focus the electron beam spot horizontally by .DELTA.Vfo (about 1.3 kv in FIG. 5B). Furthermore, the minimum sizes of the electron beam spot in the vertical and horizontal directions are different; the horizontal minimum size of the electron beam spot is about 2.5 times greater than the vertical minimum size of the electron beam spot. The voltage difference .DELTA.Vfo is referred to as an astigmatic difference. The corrective voltage applied in a system which employs a dynamic quadruple structure and a dynamic focusing action (described later) is proportional to the astigmatic difference .DELTA.Vfo.
Since the electron beam spot comes to the focus f short of the phosphor screen in the vertical direction as described above, a halo is generated above and below the electron beam spot at the peripheral edge of the phosphor screen, as shown in FIGS. 2 and 4B. As a result, the electron beam spot is distorted due to astigmatism at the peripheral edge of the phosphor screen.
Cathode-ray tubes with non-self-convergence deflection electrodes usually have a quadruple convergence electrode disposed behind the deflection electrodes. The quadruple convergence electrode is supplied with a predetermined current in synchronism with the deflection of the electron beam by the deflection electrode. Usually, the electron beam spot in such cathode-ray tubes is also distorted at the peripheral edge of the phosphor screen in the same fashion as with the self-convergence deflection electrodes.
One solution to the above problem, employed particularly for low-cost cathode-ray tube models, is to make a portion of the electron gun rotationally asymmetrical to produce an astigmatic effect on the electron beam which is opposite to the astigmatism due to the deflection magnetic field for thereby improving the electron beam spot at the peripheral edge of the phosphor screen. Inasmuch as the generated reversal astigmatic effect is fixed, the electron beam spot is necessarily brought out of focus at the center of the phosphor screen.
On the other hand, expensive cathode-ray tube models have an electromagnetic or electrostatic quadruple element near the main lens of the electron gun. The intensity of the converging effect of the quadruple element and the intensity of the focusing effect of the main lens are varied in synchronism with the deflecting action for producing a well-focused electron beam spot on the phosphor screen. Such a system is based on a combination of a dynamic quadruple structure and a dynamic focusing action. More specifically, the intensity of the converging effect of the dynamic quadruple element and the intensity of the focusing effect of the main lens are dynamically adjusted by a circuit arrangement to improve the focus of the electron beam spot at the peripheral edge of the phosphor screen while maintaining the electron beam spot in focus at the screen center.
Actually, the above system is supplied with an AC voltage whose waveform is of a quasi-parabolic shape for improving the focus of the electron beam at the peripheral edge of the phosphor screen. Since the astigmatic difference .DELTA.Vfo is large, as described above, it is customary to add an AC voltage of about 1 kv to the focusing voltage which is normally in the range of from 5 to 10 kv. Because of the high voltage requirement, the required circuit arrangement becomes expensive.
FIG. 6 of the accompanying drawings shows one recent improvement in an electron gun. In FIG. 6, sheet-like magnets 33 (see FIG. 7 of the accompanying drawings) are attached to an outer surface of the neck of a cathode-ray tube between a fourth grid G.sub.4 and a deflection electrode 22. The sheet-like magnets 33 generate a quadruple magnetic field which produces an astigmatic effect that is opposite to the astigmatic action of a main lens 31 for reducing a deflection-induced distortion of the electron beam at the peripheral edge of the screen.
The principle of the above improved electron gun is shown in FIG. 8 of the accompanying drawings, which optically simulates the electron gun. As shown in FIG. 8, the sheet-like magnets 33 are effective in providing a quadruple convergence electrode lens 34 between the main lens 31 and the deflection electrode 32 to select a vertical image magnification M.sub.v (=b.sub.v /a.sub.v) and a horizontal image magnification M.sub.H (=b.sub.H /a.sub.H) such that M.sub.v &gt;M.sub.H.
Though the electron beam spot is slightly vertically elongate in shape at the screen center, the dynamic corrective voltage (equal to the astigmatic difference .DELTA.Vfo) applied to the fourth grid G.sub.4 is lowered, making the electron beam spot more circular in shape at the peripheral edge of the screen.
While the electron beam spot is greatly improved in shape at the peripheral edge of the screen, however, it is difficult for the quadruple convergence electrode lens 34 produced by the sheet-like magnets 33 to apply an astigmatic effect equally to three electron beams R, G, B.
Heretofore, it has been customary to adjust the spot focus mainly with respect to the electron beam G which bears the information of a green image that is more important regarding visual sensitivity than the other electron beams R, B. Therefore, the spot shapes of the electron beams R, B which bears the information of red and blue images are not improved so much.